Effect of Keystone Mine Effluent on Colonization of Stream Benthos

Abstract

Colonization cages were filled with organism-free, standardized, stone substrate and placed in the stream bed above and below the effluent from an inactive lead and zinc mine in western Colorado. A significantly higher proportion of the total individuals colonizing cages from an upstream direction were dead below the effluent compared with the controls above the effluent. The percent ''mortality'' of the mayfly, Baetis bicaudatus, the dominant species in Coal Creek, was also consistently higher below the effluent. Total numbers of live insects colonizing cages were generally higher above the effluent; but results were not as consistent as ''mortality'' data. A diversity index (H'), species richness, and equitability (J') of the samples were not reliable indices of responses to stress by the benthic community. Given cages with contaminated substrate in uncontaminted water, the community recovered remarkably quickly. Colonizing populations were indistinguishable from those of control cages with uncontaminated substrate within 9 days. We conclude that the substrate habitat below the mine effluent is unsuitable for most benthic invertebrates and that the colonization technique that allows determination of percent mortality is more consistent and reliable than conventional measures of diversity for assessing the response of benthic invertebrates to stress. If water qualitymore » can be restored to a pristine state, the benthos may rapidly recover from the effects of previous acid mine drainage stress.« less